Answer for future readers: Just download Quik which is a fork of QKSMS, still being maintained at the time of writing. Quik contains all the premium features of QKSMS+ but does not have a paywall and is free. The original QKSMS has been abandoned by it's dev (also at the time of writing) and the last major version released 3 yrs ago.

QKSMS is an SMS messaging app for Android which is open-source. There is also a paid upgrade called QKSMS+. I already wanted to buy it to support the devs anyway, but I now have a good reason to buy it. I need the premium version's ability to make backups of all my message history and export them to a different phone. However, when I went to purchase it from within the app; there only seems to be an option to purchase from the Google Play Store. The only places to get the app at all as far as I'm aware, is from either Google Play, F-Droid, or the Github. I personally got it from F-Droid, but there doesn't seem to be an option to purchase QKSMS+ from it... I'm pretty sure F-Droid doesn't have any purchases in it anyway.

So does anyone know how I purchase QKSMS+ without paying through Google? If the + versions source-code is not open, I can probably get it that way. But I don't know how to compile the code manually on my phone and would rather downloading it from the source be a last resort.